Binding agent, core sand mixture and a method for producing the same

ABSTRACT

A binding agent system based on water glass is disclosed, comprising an aqueous alkali silicate solution having the general composition: xSiO 2 ·yM 2 O ·zH 2 O, with M standing for the alkali ions Li + , K + , or Na + , and a hygroscopic base, which is added in the ratio 1:4 to 1:6, with the modulus Na 2 O/SiO 2  being equal to 2.5 to 3.5 at a proportion of solids of 20 to 40% and with 30% sodium hydroxide in an aqueous solution being used as the hygroscopic base. According to the present invention, the aqueous alkali silicate solution contains an emulsion solution having 8 to 10% silicone oil in relation to the quantity of binder, with the silicone oil having a boiling point≧250°. The quantity of binder for a core sand mixture is 1.0 to 2.5%, in relation to the total quantity of granular solid, with the production method being performed in a core shooter comprising a vertically positioned housing having a horizontal air intake. The core sand mixture is introduced via a funnel into a slotted sand cylinder. The connection between the funnel and the sand cylinder is closed during the shooting. The core sand mixture located in the slotted sand cylinder is compacted using an air pressure of p 1  and then shot into the core box at a pressure p 2 &gt;p 1 .

The present invention relates to a binding agent system based on water glass, comprising an aqueous alkali silicate solution having the general composition: xSiO₂·yM₂O·zH₂O, with M standing for the alkali ions Li⁺, K⁺, or Na⁺, and a hygroscopic base, which is added in the ratio 1:4 to 1:6, with the modulus Na₂O/SiO₂ being equal to 2.5 to 3.5 at a proportion of solids of 20 to 40% and with 30% sodium hydroxide in an aqueous solution being used as the hygroscopic base, as well as a core sand mixture and a method for its production.

The binder contained in the binding agent system is used to produce core moldings for foundry purposes. Binders are a component of a molding material which, under specific conditions, such as dampening or swelling and silicate formation, produces the bond between the individual grains of sand. In addition to pure swelling binders and ceramic binders, chemical binders based on water glass or an artificial resin are also used, which react with one another chemically and solidify at the same time.

The known binders are either not sufficiently stable or display undesired side effects or waste products, which endanger the environment. Therefore, water glass bonded molding materials, which form solid phases via dehydration and solidify at the same time, have been shown to be a reasonable compromise. However, it is not always simple to control the solidification processes in such a way that they first occur in the core box. The danger of early solidification of the mold sand particularly arises due to the use of modern core molding machines. Therefore, the good flow properties of the sand, which are required as it is poured into the core box, are not achieved.

A binding agent for raw mixtures for producing self-hardening casting molds and casting cores is known from German Published Application 15 08 634 (Diamond Shamrock Corp.). The binding agent comprises sodium silicate, using a mole ratio Na₂O to SiO₂ of 1:1.5 to 1:3, and potassium hydroxide, which is added in the form of an aqueous solution. During the setting, hydrogen is released in an exothermic reaction, with the setting time being shortened by the addition of potassium hydroxide. An external supply of heat is not provided, with a further silicious material, e.g. ferrosilicon, having to be added, whose grain size is, however, to be below 0.15 mm. The rapid setting is therefore possible only under specific, strictly limited conditions and in the presence of multiple components, which must be controlled exactly.

To improve processability during the green sand casting method of water glass-bentonite or water glass-clay mixtures, 1.0 to 2.0% sodium hydroxide is added to a molding material mixture according to German Democratic Republic Patent 82 809. The molding material comprises, in addition to 2.0 to 5.0% sodium silicate, 0.5 to 1.0% starch and 2.0 to 4.0% clay, with the remainder quartz sand. The molding mass is applied to the model, backfilled with filler sand, and the finished green mold is chemically or thermally solidified. Therefore, these are green sand molds which are not suitable for mechanical core production. The sample bodies produced from this are hardened by esterification while being dried for 1 hour at 200° C. and then subjected to a compression test, with a maximum strength of 500 to 600 N/m² resulting. This maximum value was only achieved with a molding material in which the surface-active components did not exceed a total quantity of 0.1%, in relation to the total quantity of the material.

A method in the production of bodies made of granular and/or fibrous material using sodium silicate or potassium silicate as a binding agent is known from German Patent Specification 29 09 107, with a surface-active material being added to the mixture of the material. The surfactant material may comprise a surface-active, a silicone oil, or a silicone emulsion, at a quantity of at most 0.1%, in relation to the total quantity of material.

The object of the present invention is to develop a binding agent system for mechanical core production through core shooting/core blowing which may be easily processed in the dry process (in the dried, pulverized state) and has an improved flowability and reduced setting time at the same time.

This object is achieved according to the present invention by the features indicated in the patent claims.

The novel binding agent system based on water glass comprises an aqueous alkali silicate solution of the general composition: xSiO₂·yM₂O·zH₂O, with M standing for the alkali ions Li⁺, K⁺, or Na⁺, and is characterized in that it additionally contains a hygroscopic base. Sodium hydroxide is preferably used as the hygroscopic base in the ratio 1:4 to 1:6. If the modulus Na₂O/SiO₂ is equal to 2.5 to 3.5 at a proportion of solids of 20 to 40%, this prevents the mold sand from solidifying prematurely under all operating conditions. 30% sodium hydroxide in an aqueous solution is used as the hygroscopic base. A surface-active material is added to control the hygroscopic properties.

If silicone oil having a boiling point≧250° is added to the binding agent system as the surface-active material, the flowability of the mold sand in the core shooter may be increased significantly. An emulsion solution having 8 to 10% silicone oil, in relation to the quantity of binder, is added to the binding agent system, with the following advantageous refinements being possible:

-   -   a) the silicone emulsion solution has anionic, cationic,         non-ionogenic properties,     -   b) an emulsifier is used in the emulsion solution which elevates         the viscosity of the base oil and simultaneously reduces the         surface tension of the binding agent system. The emulsifier has         a hydrophilic and a lipophilic molecule part, which projects         into the oil phase,     -   c) an oil is used as a base oil for the silicone emulsion         solution which forms a sodium oleate film in the binding agent         system, which generates a mechanically stable protective layer         in the boundary surface of the dispersion system,     -   d) the silicone oil emulsion solution has the following         structure:         -   the Si atoms are each bonded to two CH₃ groups,         -   the Si atoms are bonded to one another via oxygen atoms and             form a chain, with the CH₃ groups of the respective             neighboring Si atoms being positioned spatially displaceable             around the oxygen atoms.

In the following, the present invention is described in more detail with reference to a comparative example. In this case, a core shooter as shown in FIG. 1 is used, which has the following design:

The core shooter comprises a vertically positioned housing A, onto which a horizontal air intake B is flange-mounted. The air intake is controlled via a large-surface air intake valve 1, with an elevated air pressure arising in air supply tank 5. When air intake valve 1 is opened, the air pressure reaches engine chamber 10 of housing A, in which a slotted sand cylinder 6 is positioned vertically. Sand cylinder 6 is connected to a sand supply container 3, which may be emptied via a slide valve 4.

If the air intake valve is operated, sand is conveyed from slotted sand cylinder 6 into shooting head 7 having a sand outlet opening and enters core box 8, with a core sand aspirator 9 a, 9 b ensuring the production of compacted core sand 11.

To assess the moisture content of the mold sand, a moisture meter is integrated into air exit valve 2. Using this arrangement, it was determined that with the implementation of the core sand according to the present invention, significantly higher moisture contents, and therefore better viscosity and better flowability, of the mold sand could be achieved.

The core sand mixture according to the present invention contains a quantity of binder from 1.0 to 2.5%, in relation to the total quantity of granular solid. The granular solid is preferably quartz sand having a grain size of 0.1 to 0.3 mm. The quantity of sodium hydroxide added to the binding agent system is 0.01 to 0.6% and 0.1 to 0.5% silicone oil is contained therein. In the following, a compressive strength test is performed using the binding agent according to the present invention.

Series 1, charge makeup:

-   2.4% binder, 0.4% emulsion, remainder quartz sand -   Sample weight 158 g, sample body—standard height 50 mm 2 minutes of     microwave drying after removal from the core shooter. -   Testing after 30 minutes at room temperature. -   Compressive strength: average value from 5 tests 1060 N/cm²

Series 2, charge makeup:

-   2.4% binder, 0.4% NaCH, 0.2% silicone oil, remainder quartz sand -   Sample weight: 157 g, sample body—standard height 50 mm 2 minutes of     microwave drying after removal from the core shooter. -   Compressive strength testing after 30 minutes at room temperature. -   Average resistance value from 5 tests 1164 N/cm².

This comparative experiment showes that the binding agent systems according to the present invention may be processed into high-strength mold bodies using a typical core shooter. The test bodies had an approximately 10% improvement in compressive strength if 0.2% silicone oil was added to the test mixture. If the method according to the present invention according to claim 1 was used, the comprehensive strength could be increased even more. 

1. A binding agent for a core sand mixture used in foundry processes, comprising: (a) a binder incorporating an aqueous alkali silicate solution containing
 1. an alkali silicate having the formula xSiO₂·yM₂O·zH₂O wherein M is Li⁺, K⁺, or Na⁺; and
 2. a hygroscopic base, sodium hydroxide, in the ratio of one part of the base per 4 to 6 parts of the alkali silicate;  wherein the ratio of Na₂O/SiO₂ is 2.5 to 3.5 and the solids content is 20 to 40% of the solution; and, in admixture therewith; (b) an emulsion containing a silicone oil having a boiling point of ≧250° in an amount of from 8 to 10% of the binder.
 2. The binding agent according to claim 1, wherein the silicone oil has the following structure: (c) Si atoms are each bonded to two CH₃ groups, (d) Si atoms are bonded to one another via oxygen atoms and form a chain wherein the CH₃ groups of the respective neighboring Si atoms are positioned spatially displaced around the oxygen atoms.
 3. A core sand mixture for foundry purposes, comprising sand and the binding agent of claim 1 in an amount of 1.0 to 2.5% of the total quantity of sand.
 4. The core sand mixture of claim 3, wherein the sand is quartz sand having a grain size from 0.1 to 0.3 mm.
 5. The core sand mixture of claim 3, wherein the quantity of sodium hydroxide in the binding agent is 0.01 to 0.6% thereof.
 6. The core sand mixture of claim 3, wherein the quantity of silicone oil in the binding agent is 0.1 to 0.5% thereof.
 7. A method of producing a core sand mixture for foundry purposes, comprising the steps of: (e) introducing the core sand mixture of claim 3 into a vertically positioned slotted sand cylinder; (f) subjecting the core sand mixture within the slotted sand cylinder to elevated air pressure pi to compress the core sand mixture; and (g) feeding the compressed core sand mixture through a core shooter and into a core box at a pressure p₂>p₁ to provide the core sand mixture.
 8. The method of claim 7, where in step (c) the pressure p₂ within the core box is relieved by aspiration to release pressurized air and moisture. 